A method of injection molding parts, including: injecting a first thermoplastic polymer into a first cavity; forming a ribbed structure comprising ribs in the first cavity, wherein each rib in the ribbed structure includes a microstructure on an outer portion of a rib; and reducing the surface imperfections in the part by overmolding a layer formed in the second cavity onto a portion of a rib by injecting a second thermoplastic polymer into the second cavity, wherein the overmolding occurs at an interface between the layer and the ribbed structure; or injecting a first thermoplastic polymer into a first cavity; forming a layer in the first cavity; and overmolding a ribbed structure comprising ribs formed in the second cavity onto a portion of the layer formed in the first cavity. In further variations gas is injected into a microchannel formed at the interface or a foaming agent is applied in the first thermoplastic polymer.